NishitaAtmos.osl

#include <stdosl.h>

int has_solutions (vector StartPoint, vector UnitVector, float Radius) {
	float b = 2 * dot (UnitVector, StartPoint);
	float bsquared = pow(b,2);
	float c = dot (StartPoint,StartPoint) - pow(Radius,2);
	float fourac = 4 * c;

	if (bsquared > fourac) {
		return 1;
	} else {
		return 0;
	}
}

int linetype (vector StartPoint, vector UnitVector, float Radius) {
	float b = 2 * dot (UnitVector, StartPoint);
	float bsquared = pow(b,2);
	float c = dot (StartPoint,StartPoint) - pow(Radius,2);
	float fourac = 4 * c;

	if (bsquared > fourac) {
		float d1 = (-b + pow(bsquared - fourac, 0.5)) / 2;
		float d2 = (-b - pow(bsquared - fourac, 0.5)) / 2;
		if (d1 < 0 && d2 < 0) {
			return 2;
		} else if (d1 < 0 || d2 < 0) {
			return 1;
		} else {
			return 0;
		}
	} else {
		return 3;
	}
}

vector lmin (vector StartPoint, vector UnitVector, float Radius) {
	float b = 2 * dot (UnitVector, StartPoint);
	float bsquared = pow(b,2);
	float c = dot (StartPoint,StartPoint) - pow(Radius,2);
	float fourac = 4 * c;

	if (bsquared > fourac) {
		float d1 = (-b + pow(bsquared - fourac, 0.5)) / 2;
		float d2 = (-b - pow(bsquared - fourac, 0.5)) / 2;
		return StartPoint + UnitVector * min(d1, d2);
	} else {
		return vector(0);
	}
}

vector lmax (vector StartPoint, vector UnitVector, float Radius) {
	float b = 2 * dot (UnitVector, StartPoint);
	float bsquared = pow(b,2);
	float c = dot (StartPoint,StartPoint) - pow(Radius,2);
	float fourac = 4 * c;

	if (bsquared > fourac) {
		float d1 = (-b + pow(bsquared - fourac, 0.5)) / 2;
		float d2 = (-b - pow(bsquared - fourac, 0.5)) / 2;
		return StartPoint + UnitVector * max(d1, d2);
	} else {
		return vector(0);
	}
}

struct linesample {
	int has_solutions;
	int type;
	vector min;
	vector max;
};

shader nishita_sky (
	float Height = 1,
	float Intensity = 20,
	//float Hazyness = 0.5
	//	[[float min = 0.0, float max = 2.0]],

	vector SunVector = vector(0,0,1),
	vector Vector = P,
	output color Rayleigh = 0.0,
	output color Mie = 0.0,
	output color Sky = 0.0)

{
	//Variables:
	float Re = 6360e3; // Radius of the earth
	float Ra = 6420e3; // Radius of the atmosphere
	float Hr = 7994; // Rayleigh scale height
	float Hm = 1200; // Mie scale height
	float g = 0.76; // Anisotropy term for Mie scattering.

	vector BetaR = vector(5.8e-6,13.0e-6,22.4e-6);
	vector BetaM_max = vector(9.73e-6);
	vector BetaM_min = vector(13.28e-6);
	//vector BetaM = BetaM_min + Hazyness * (BetaM_max - BetaM_min);
	vector BetaM =  vector(20e-6);


	//Clean up inputs:
	vector SunDirection = normalize(SunVector);
	vector Direction = normalize(Vector);

	//Calcualte Rayleigh and mie phases.
	float mu = dot(Direction, SunDirection);
  	float phaseR = (3/(16 * M_PI)) * (1 + mu*mu);
  	float phaseM = (3/(8 * M_PI)) * ((1 - g*g) * (1 + mu*mu) / ((2 + g*g) * pow(1 + g*g - 2 * g * mu, 1.5)));

	//Stuff that will eventually help form my output.
	vector SumR = vector(0);
	vector SumM = vector(0);

	//Set up start and end points for my integrals.
	vector Pu = vector(0,0,0);
	vector Pv = vector(0,0,0);

	//Get start and end point for View Line.
	if (Height > 0) {
		vector Pc = vector (0,0,Re + Height);
		Pu = Pc;
		
		linesample groundline;
		groundline.has_solutions = has_solutions(Pc, Direction, Re);
		groundline.type = linetype(Pc, Direction, Re);
		groundline.min = lmin(Pc, Direction, Re);
		groundline.max = lmax(Pc, Direction, Re);

		linesample atmosphereline;
		atmosphereline.has_solutions = has_solutions(Pc, Direction, Ra);
		atmosphereline.type = linetype(Pc, Direction, Ra);
		atmosphereline.min = lmin(Pc, Direction, Ra);
		atmosphereline.max = lmax(Pc, Direction, Ra);

		Pv = atmosphereline.max;
		if (atmosphereline.has_solutions && atmosphereline.type == 0) {
			//Line starts outside atmoshere, so Pu becomes Line.min
			Pu = atmosphereline.min;
		}
		if (groundline.has_solutions && groundline.type == 0) {
			//Line hits ground, Pv becomes point at which line hits ground, which will be Line.min
			Pv = groundline.min;
		}
	}

	// Create samples along view ray.
	int numSamples = 16;
	int numSamplesL = 8;
	
	float segmentLength = distance (Pu, Pv) / numSamples;
	float opticalDepthR = 0;
  	float opticalDepthM = 0;

	for (int i = 0; i < numSamples; i++) {
	vector Px = Pu + segmentLength * Direction * (i + 0.5);
	float sampleHeight = length(Px) - Re;

		/* Get optical depth for light at this sample: */
		float Hr_sample = exp(-sampleHeight/Hr) * segmentLength;
		float Hm_sample = exp(-sampleHeight/Hm) * segmentLength;

		opticalDepthR += Hr_sample;
		opticalDepthM += Hm_sample;

		//Get light depth to sun at this sample.
		float opticalDepthLR = 0;
		float opticalDepthLM = 0;

		linesample atmosphereline_sample;
		atmosphereline_sample.has_solutions = has_solutions(Px, SunDirection, Ra);
		atmosphereline_sample.type = linetype(Px, SunDirection, Ra);
		atmosphereline_sample.min = lmin(Px, SunDirection, Ra);
		atmosphereline_sample.max = lmax(Px, SunDirection, Ra);

		linesample groundline_sample;
		groundline_sample.has_solutions = has_solutions(Px, SunDirection, Re);
		groundline_sample.type = linetype(Px, SunDirection, Re);
		groundline_sample.min = lmin(Px, SunDirection, Re);
		groundline_sample.max = lmax(Px, SunDirection, Re);

		vector Ps = atmosphereline_sample.max;
		int j = 0;
		if (groundline_sample.has_solutions && groundline_sample.type == 0) {
			//Light is below horizon from this point. No sample needed.
			opticalDepthLR += 0;
			opticalDepthLM += 0;
		} else {
			float segmentLengthL = distance(Px, Ps) / numSamplesL;

			for (j = 0; j < numSamplesL; j++) {
				vector Pl = Px + segmentLengthL * SunDirection * (j + 0.5);
				float sampleHeightL = length(Pl) - Re;
				if (sampleHeightL < 0) break;
				// ignore sampleheights < 0 they're inside the planet...
				float Hlr_sample = exp(-sampleHeightL/Hr)*segmentLengthL;
				float Hlm_sample = exp(-sampleHeightL/Hm)*segmentLengthL;

				opticalDepthLR += Hlr_sample;
				opticalDepthLM += Hlm_sample;
			}
		}

		// With our light samples done we can calculate the attenuation.
		// Only include a sample if we reached j (so not if we hit the break clause for rays that hit the ground in finding the sun)
		if (j == numSamplesL) {
			vector tauR = BetaR * (opticalDepthR + opticalDepthLR);
			vector tauM = BetaM * 1.1 * (opticalDepthM + opticalDepthLM);
			vector tau = tauR + tauM;
			vector attnR = vector (exp(-tauR[0]), exp(-tauR[1]), exp(-tauR[2]));
			vector attnM = vector (exp(-tauM));
			vector attenuation = vector (exp(-tau[0]), exp(-tau[1]), exp(-tau[2]));

			SumR += Hr_sample * attenuation;
			SumM += Hm_sample * attenuation; 
		}
	}

	Rayleigh = color (SumR * phaseR * BetaR) * Intensity;
	Mie = color (SumM * phaseM * BetaM) * Intensity;
	Sky =  Rayleigh + Mie;
}